Entity%20Relationship%20Diagrams

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Entity Relationship Diagrams

• Basic Elements and Rules

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Introduction to Entity-Relationship (E-R) Modeling

• Notation uses three main constructs
• Data entities
• Relationships
• Attributes
• Entity-Relationship (E-R) Diagram
• A detailed, logical representation of the
  entities, associations and data elements for an
  organization or business

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Entity-Relationship (E-R) ModelingKey Terms

• Entity
• A person, place, object, event or concept in the
  user environment about which the organization
  wishes to maintain data
• Represented by a rectangle in E-R diagrams
• Entity Type
• A collection of entities that share common
  properties or characteristics
• Attribute
• A named property or characteristic of an entity
  that is of interest to an organization

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Entity-Relationship (E-R) ModelingKey Terms

• Candidate keys and identifiers
• Each entity type must have an attribute or set of
  attributes that distinguishes one instance from
  other instances of the same type
• Candidate key
• Attribute (or combination of attributes) that
  uniquely identifies each instance of an entity
  type

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Examples

• Identify a few entity types, instances,
  attributes and candidate keys for
• DePaul Campus Connect Registration System
• Illinois Bureau of Motor Vehicles System
• Amazon.com Product Information System

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Depicting Entities and Attributes

• Draw a portion of the ERD for each of these
  systems
• DePaul Campus Connect Registration System
• Illinois Bureau of Motor Vehicles System
• Amazon.com Product Information System

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Conceptual Data Modeling and the E-R Diagram

• Goal
• Capture as much of the meaning of the data as
  possible
• If you know the rules of normalization,
  referential integrity, foreign keys, etc., this
  is good but not as important now. It is much
  more important to get the organizational data
  model correct, i.e. to understand the actual data
  requirements for the organization.
• Result
• A better design that is scalable and easier to
  maintain

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Entity-Relationship (E-R) ModelingKey Terms

• Identifier
• A candidate key that has been selected as the
  unique identifying characteristic for an entity
  type
• Selection rules for an identifier
• Choose a candidate key that will not change its
  value
• Choose a candidate key that will never be null
• Avoid using intelligent keys
• Consider substituting single value surrogate keys
  for large composite keys

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Entity-Relationship (E-R) ModelingKey Terms

• Relationship
• An association between the instances of one or
  more entity types that is of interest to the
  organization
• Association indicates that an event has occurred
  or that there is a natural link between entity
  types
• Relationships are always labeled with verb phrases

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Cardinality

• The number of instances of entity B that can be
  associated with each instance of entity A
• Minimum Cardinality
• The minimum number of instances of entity B that
  may be associated with each instance of entity A
• This is also called modality.
• Maximum Cardinality
• The maximum number of instances of entity B that
  may be associated with each instance of entity A

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Naming and Defining Relationships

• Relationship name is a verb phrase
• Avoid vague names
• Guidelines for defining relationships
• Definition explains what action is being taken
  and why it is important
• Give examples to clarify the action
• Optional participation should be explained
• Explain reasons for any explicit maximum
  cardinality

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Naming and Defining Relationships

• Guidelines for defining relationships
• Explain any restrictions on participation in the
  relationship
• Explain extent of the history that is kept in the
  relationship
• Explain whether an entity instance involved in a
  relationship instance can transfer participation
  to another relationship instance

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Common Modeling Situations

• Unary relationships
• Binary relationships
• Ternary relationships
• Attributive or Weak Entities
• Associative or Intersection Entities
• Supertypes and Subtypes

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Degree of Relationship

• Degree
• Number of entity types that participate in a
  relationship
• Three cases
• Unary
• A relationship between two instances of one
  entity type
• Binary
• A relationship between the instances of two
  entity types
• Ternary
• A simultaneous relationship among the instances
  of three entity types
• Not the same as three binary relationships

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Figure 10-6Example relationships of different
degrees